Non-aqueous electrolyte battery and battery pack

ABSTRACT

The non-aqueous electrolyte battery includes an outer case, a positive electrode housed in the outer case, a negative electrode housed in the outer case such that the negative electrode is separated from the positive electrode, and a non-aqueous electrolyte accommodated in the outer case. The negative electrode comprises a current collector and negative electrode layer formed on one surface or both surfaces of the current collector. The negative electrode layer includes at least one main negative electrode layer which is formed on the surface of the current collector and contains a first active material, and a surface layer which is formed on the surface of the main negative electrode layer and contains a second active material different from the first active material, the second active material being a lithium titanium composite oxide having a spinel structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromprior Japanese Patent Application No. 2008-265401, filed Oct. 14, 2008,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a non-aqueous electrolyte battery and abattery pack.

2. Description of the Related Art

A non-aqueous electrolyte battery that effects charging/dischargingthrough the movement of lithium ions between a negative electrode and apositive electrode is intensively studied and developed as a high energydensity battery.

The non-aqueous electrolyte battery is demanded to have variousproperties depending on the end-use thereof. For example, if it isdesired to be used as a power source for a digital camera, the batteryis required to exhibit the discharging of about 3 C. Whereas, if it isdesired to be used as a power source for vehicles such as a hybridelectric motor car, the battery is expected to require the dischargingof about 10 C or more. For this reason, the non-aqueous electrolytebattery to be used for these end-uses is desired to exhibit especiallylarge current characteristics.

In the non-aqueous electrolyte battery available in the market,lithium-transition metal composite oxides are employed as an activematerial of the positive electrode and carbonaceous materials areemployed as an active material of the negative electrode. As for thetransition metals included in the lithium-transition metal compositeoxides, metals such as Co, Mn, Ni, are generally employed.

In recent years, there is concern about the degradation in safety of thenon-aqueous electrolyte battery, attributable to improvements for higheroutput power, higher energy density and higher capacity.

With respect to the safety of the non-aqueous electrolyte, internalshort-circuit caused by contamination with electroconductive foreignsubstances in a production process of the batteries cannot be preventedby an external circuit (protection circuit) devised to prevent overcharging/over discharging. Therefore, this problem should be coped withby the battery itself.

Accordingly, JP-A 2005-183179 (KOKAI) discloses a battery wherein aninorganic insulating material layer such as an alumina layer is formedon the surface of a negative electrode containing a carbonaceousmaterial as an active material to improve safety at the time of internalshort-circuit.

In the invention disclosed in the above document, however, the inorganicinsulating material layer such as an alumina layer formed on the surfaceof the negative electrode is highly resistive and functions as aresistance component against the negative electrode regardless of acharging/discharging state, thus degrading the large-current performanceof the battery.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided anon-aqueous electrolyte battery comprising: an outer case; a positiveelectrode housed in the outer case; a negative electrode housed in theouter case with a space from the positive electrode; and a non-aqueouselectrolyte accommodated in the outer case, wherein the negativeelectrode comprises a current collector and an activematerial-containing negative electrode layer formed on one side or bothsides of the current collector, and the negative electrode layerincludes at least one main negative electrode layer which is formed onthe surface of the current collector and contains a first activematerial, and a surface layer which is formed on the surface of the mainnegative electrode layer and contains a second active material differentfrom the first active material, the second active material being alithium titanium composite oxide having a spinel structure.

According to a second aspect of the invention, there is provided anon-aqueous electrolyte battery comprising: an outer case; a positiveelectrode housed in the outer case; a negative electrode housed in theouter case with a space from the positive electrode; and a non-aqueouselectrolyte accommodated in the outer case, wherein the negativeelectrode comprises a current collector and a negative electrode layerformed on one side or both sides of the current collector, the negativeelectrode layer includes at least one main negative electrode layerwhich is formed on the surface of the current collector and contains afirst active material, and a surface layer which is formed on thesurface of the main negative electrode layer and contains a secondactive material different from the first active material, the secondactive material absorbs and releases lithium, a volume resistivity ofthe second active material in a lithium-nonabsorbed state is 1×10⁵ Ωcmor more, and the volume resistivity of the second active material in alithium-absorbed state is 1×10⁻² times or less relative to the volumeresistivity thereof in a lithium-nonabsorbed state.

According to a third aspect of the invention, there is provided abattery pack including a plurality of the non-aqueous electrolytebatteries in the first aspect, wherein the batteries are electricallyconnected each other in series, in parallel, or in series and parallel.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a sectional view of a flattened non-aqueous electrolytebattery according to an embodiment.

FIG. 2 is an enlarged sectional view of part A in FIG. 1.

FIG. 3 is an exploded perspective view of a battery pack according tothe embodiment.

FIG. 4 is a block diagram of the battery pack in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, the non-aqueous electrolyte battery and the battery packaccording to the embodiment of the present invention will be explainedin detail.

This non-aqueous electrolyte battery in accordance with the embodimentis equipped with an outer case. A positive electrode is housed in theouter case. A negative electrode is housed in the outer case with aspace from the positive electrode, for example, with a separator beinginterposed between them. A non-aqueous electrolyte is accommodated inthe outer case.

Now, the respective members of the non-aqueous electrolyte battery inthe embodiment are described in detail.

(1) Negative Electrode

The negative electrode comprises a current collector and a negativeelectrode formed on one surface or both surfaces of the currentcollector and containing an active material. The negative electrodelayer is a laminate of a plurality of layers containing different activematerials. That is, the negative electrode layer has a multilayerstructure including at least one main negative electrode layer which isformed on the surface of the current collector and contains a firstactive material, and a surface layer which is formed on the surface ofthe main negative electrode layer and contains a second active materialdifferent from the first active material. The second active materialabsorbs and releases lithium. The volume resistivity of the secondactive material is 1×10⁵ Ωcm or more in a lithium-nonabsorbed state, andthe volume resistivity of the second active material in alithium-absorbed state is 1×10⁻² times or less relative to the volumeresistivity thereof in a lithium-nonabsorbed state. The volumeresistivity of the second active material in a lithium-absorbed state ismore preferably 1×10⁻⁴ times or less relative to the volume resistivityof the second active material in a lithium-nonabsorbed state.

The phrase “a volume resistivity of the second active material in alithium-nonabsorbed state” refers to the volume resistivity inherent inthe second active material into which lithium is not absorbed.

When the non-aqueous electrolyte battery including a negative electrodehaving a surface layer containing a second active material having theproperty described above is charged and discharged for the initial time,the second active material may, even after discharge to 0V, have lithiumremaining therein. That is, when a negative electrode having a surfacelayer containing the second active material having the propertydescribed above is integrated in a battery and then charged anddischarged for the initial time, the second active material comes tohave lithium substantially absorbed therein, so that when the secondactive material removed from the surface layer after discharge to 0V ismeasured for its volume resistivity, the resistivity is a value lowerthan the volume resistivity of the second active material in alithium-nonabsorbed state. It follows that when the battery afterdischarge to 0V is dismantled and the second active material is removedfrom the surface layer of the negative electrode and subjected toinactivation treatment, then the volume resistivity of the second activematerial in a lithium-nonabsorbed state can be measured.

The volume resistivity of the second active material can be measured inthe following manner.

Plate-shaped lower and upper steel electrodes each having, in the centerof one side thereof, a vertically protruding cylindrical body having anouter diameter of 1 inch, and an insulating tube made of a vinylchloride resin having an inner diameter of 1 inch, are preparedrespectively. The total length of the cylindrical bodies of the lowerand upper electrodes is the same as the length of the insulating tube.

On a plate-shaped insulating sheet of 2 mm in thickness consisting ofpolytetrafluoroethylene, the lower electrode is placed such that itscylindrical body protrudes upward. The insulating tube is connected tothe cylindrical body of the lower electrode so that the bottom of theinsulating tube is positioned on the upper surface of the cylindricalbody. 5 g of a sample weighed out with an even balance is filled in theinsulating tube. The cylindrical body of the upper electrode is insertedinto the opening at the upper end of the insulating tube. A plate-shapedinsulating sheet consisting of polytetrafluoroethylene is placed on theupper surface of the upper electrode. Using a hydraulic press machine,load is applied to the plate-shaped insulating sheet at the upper side,so that the sample in the insulating tube is pressed with thecylindrical body of the upper electrode inserted into the insulatingtube. The load with the hydraulic press machine is set at 10 kg/cm² on ahydraulic gauge scale. An LCR meter (resistance meter) (trade name:AR-480D manufactured by Keisei Corporation) is connected to the lowerand upper electrodes under pressure at 10 kg/cm², and immediately afterconnection, the resistance r (Ω) is read. The thickness (cm) of thesample pressed in the insulating tube is measured, and the volumeresistivity (Ωcm) of the sample is then calculated. An equation forcalculating the volume resistivity is as follows:

Volume resistivity (Ωcm)={(2.54/2)²×π}×(r/L)

wherein r is the resistance immediately after connection, and L is thethickness of the sample after filled and pressed in the insulating tube.

Besides the LCR meter (trade name: AR-480D manufactured by KeiseiCorporation), a resistance meter having equivalent or better performancemay also be used.

When the negative electrode including a surface layer containing asecond active material having such property generates internalshort-circuit in the positive electrode/negative electrode-opposedregion, the second active material serving as a short-circuit part inthe surface layer shows insulation properties, thus making the passageof a large current difficult. As a result, the battery can be preventedfrom generating heat, and safety can be improved.

That is, the surface layer of the negative electrode layer in thenegative electrode contains a second active material having a volumeresistivity of 1×10⁵ Ωcm or more in a lithium-nonabsorbed state, and thevolume resistivity of the second active material in a lithium-absorbedstate is 1×10⁻² times or less relative to the volume resistivity thereofin a lithium-nonabsorbed state. In Other words, this second activematerial in the lithium-nonabsorbed state has a volume resistivity of1×10⁵ Ωcm or more, and substantially shows insulation properties. Thesecond active material in the lithium-absorbed state has the volumeresistivity of 1×10⁻² times or less relative to the volume resistivitythereof in the lithium-nonabsorbed state, and shows relatively goodconductive property. When the non-aqueous electrolyte battery includinga negative electrode having such a surface layer generates internalshort-circuit, for example due to foreign substance present in thepositive electrode/negative electrode opposed region, the foreignsubstance is contacted with the surface layer of the negative electrode.The contact of the foreign substance with the surface layer and thegeneration of internal short-circuit lead to rapid discharge in the partof contact of the second active material in the surface layer with theforeign substance, to bring about a lithium-unabsorbed state of thesecond active material. Accordingly, the surface layer containing thesecond active material positioned in the short-circuit part where thesecond active material is contacted with the foreign substance comes tohave a volume resistivity of 1×10⁵ Ωcm or more, and substantially showsinsulation properties. As a result, the passage of a current via theforeign substance between the positive electrode and the negativeelectrode is limited by the second active material showing insulationproperties, thus making the passage of a large current difficult.Consequently, the battery can be prevented from generating heat andsafety can be improved.

A lithium titanium composite oxide having a spinel structure for examplecan be used as the second active material contained in the surfacelayer. When this lithium titanium composite oxide is used, itsinhibitory action on passage of a current via foreign substance betweenthe positive electrode and the negative electrode is specificallydescribed.

The lithium titanium composite oxide having a spinel structure (forexample, Li₄Ti₅O₁₂) has a lithium working electric potential of about1.55V (vs. Li/Li⁺), and absorbs and releases lithium upon batterycharging/discharging as shown in the following formula 1:

Li₄Ti₅O₁₂+3Li⁺+3e⁻

Li₇Ti₅O₁₂   (1)

In formula 1, the rightward arrow shows charging, and the leftward arrowshows discharging.

As shown in formula 1, Li₄TiSO₁₂ in a lithium-unabsorbed state has avolume resistivity of about 1×10⁶ Ωcm and substantially shows insulationproperties. On the other hand, the lithium titanium composite oxide in alithium-absorbed state has a volume resistivity of about 1×10¹ to 1×10²Ωcm, that is, about 1×10⁻⁴ to 1×10⁻⁵ times relative to that ofLi₄Ti₅O₁₂, and shows conductive property.

When the non-aqueous electrolyte battery including a negative electrodehaving a surface layer containing such a lithium titanium compositeoxide generates internal short-circuit for example due to foreignsubstance present in the positive electrode/negative electrode-opposedregion, the foreign substance is contacted with the surface layer of thenegative electrode. The contact of the foreign substance with thesurface layer and the generation of internal short-circuit lead to rapiddischarge in the part of contact, with the foreign substance, of thelithium titanium composite oxide having a spinel structure in thesurface layer, to bring about a lithium-unabsorbed state of the lithiumtitanium composite oxide. Accordingly, the surface layer containing thelithium titanium composite oxide having a spinel structure positioned inthe short-circuit part where the composite oxide is contacted with theforeign substance comes to have a volume resistivity of about 1×10⁶ Ωcm,and substantially shows insulation properties. As a result, the passageof a current via the foreign substance between the positive electrodeand the negative electrode is limited by the surface layer containingthe lithium titanium composite oxide showing insulation properties, thusmaking the passage of a large current difficult. As a result, thebattery can be prevented from generating heat and safety can beimproved.

The second active material with the above property (for example, thelithium titanium composite oxide having a spinel structure) contained inthe surface layer of the negative electrode effects usualcharging/discharging by absorption and release of lithium, except uponinternal short-circuit accompanying its contact with foreign substanceor the like, that is, except while in a lithium-unabsorbed stateoccurring in the part of contact with foreign substance or the like toincrease the volume resistivity. That is, the second active material hasabsorbed lithium therein, and shows relatively good conductive propertyas compared with the second active material in a lithium-unabsorbedstate. As a result, the surface layer of the negative electrode layer ina usually charged/discharged state, unlike a conventional inorganicinsulating material layer (for example, an alumina layer), does notfunction as a resistance component against the negative electrode, andcan thus maintain large current performance.

The second active material contained in the surface layer of thenegative electrode layer, for example the lithium titanium compositeoxide having a spinel structure, is preferably Li_(4+x)Ti₅O₁₂ (−1≦x≦3),from the viewpoint of the reversibility of charging/discharging(charge/discharge cycle performance). Although the molar ratio oflithium titanium composite oxide is formally represented by 12 in thespinel type Li_(4+x)Ti₅O₁₂ (−1≦x≦3), these values may fluctuatedepending on the influence of oxygen nonstoichiometry. Even ifinevitable impurities are contained, the effect of the present inventionis not lost.

The average thickness of the surface layer is preferably 3 μm to 30 μm.The average thickness of the surface layer can be measured by thefollowing method. The concentration distribution of a main component ofthe second active material contained in the surface layer is measured ata plurality of points (arbitrary 5 points or more) on a section of thesurface layer of the negative electrode. For example, with SEM-EDX orthe like, a line profile of titanium is observed in the direction oflamination of the layer, and its flexion point is assumed to be in theinterface between the surface layer and the main negative electrodelayer, to determine the thickness of the surface layer. When the secondactive material contained in the surface layer is a lithium titaniumcomposite oxide having a spinel structure, the concentrationdistribution of titanium or oxygen can be measured to determine thethickness of the surface layer.

When the thickness of the surface layer is less than 3 μm, it becomesdifficult to exhibit an excellent current blocking effect duringinternal short-circuit attributable to foreign substance. When thethickness of the surface layer is more than 30 μm, the proportion of thesurface layer in the negative electrode layer is increased, that is, theproportion of the main negative electrode layer is decreased, resultingsometimes in hindrance to achieve a higher capacity/higher energydensity of the battery. That is, when the second active material is alithium titanium composite oxide having a spinel structure, itstheoretical electric capacity is 175 mAh/g. When the electric capacityof the second active material contained in the main negative electrodelayer is higher than that of the spinel-type lithium titanium compositeoxide, an increase in the thickness of the surface layer containing thespinel-type lithium titanium composite oxide hinders achievement of ahigher capacity/higher energy density of the battery.

The surface layer containing the second active material having aproperty of drastically changing its volume resistivity with changebetween a lithium-unabsorbed state and a lithium-absorbed state, forexample, the lithium titanium composite oxide having a spinel structure,includes (1) a layer consisting exclusively of the lithium titaniumcomposite oxide, (2) a layer consisting of a mixture of the lithiumtitanium composite oxide and a binder, and (3) a layer consisting of amixture of the lithium titanium composite oxide, a conductive material,and a binder.

The surface layer of the lithium titanium composite oxide having aspinel structure as described in item 1, above, can be formed by, forexample, a dry coating method such as CVD or sputtering. The surfacelayer containing the lithium titanium composite oxide having a spinelstructure as described in items 2 and 3, above, can be prepared forexample by dispersing the lithium titanium composite oxide having aspinel structure, together with the binder or the binder and theconductive material, in a solvent such as N-methylpyrrolidone (NMP) toprepare slurry, and then applying and drying the slurry. This coatingmethod can form a thick film industrially in a short time.

The binder contained in the surface layers described in items 2 and 3,above, includes, for example, polytetrafluoroethylene (PTFE),polyvinylidene fluoride (PVdF), fluorine-based rubber, styrene butadienerubber, a core-shell binder, or the like. The binder is compounded in anamount of preferably 15% by weight or less, more preferably 6% by weightor less, based on the weight of the active material in the surfacelayer.

The conductive material contained in the surface layer described in item3, above, includes, for example, carbon materials such as acetyleneblack, carbon black, cokes, carbon fibers or graphite, electroconductiveceramic powder such as powder of titanium nitride or titanium oxide, andmetal powder such as aluminum powder. The conductive material iscompounded in an amount of preferably 15% by weight or less, morepreferably 6% by weight or less, based on the weight of the activematerial in the surface layer.

The main negative electrode layer in the negative electrode layer has astructure of one or more layers each containing a first active materialdifferent from the second active material in the surface layer.

The lithium absorption-release potential of the first active materialcontained in the main negative electrode layer is preferably lower than3V (vs. Li/Li⁺). When the lithium absorption-release potential is higherthan 3V (vs. Li/Li⁺), the voltage of the battery may be decreased tolower the energy density of the battery. From this viewpoint, the firstactive material is selected preferably from carbonaceous materials,lithium titanium composite oxides (excluding those having a spinelstructure), lithium molybdenum composite oxides, or lithium niobiumcomposite oxides.

On the other hand, the lithium absorption-release potential of the firstactive material is preferably higher than 1.0V (vs. Li/Li⁺). The lithiumabsorption-release potential of the second active material (for example,the lithium titanium composite oxide having a spinel structure)contained in the surface layer is 1.55V (vs. Li/Li⁺). When the lithiumabsorption-release potential of the first active material is made higherthan 1.0V (vs. Li/Li⁺), the difference of the lithium absorption-releasepotential thereof from that of the lithium titanium composite oxide isdecreased so that the reversibility of charging/discharging in thesurface layer can be improved. From this viewpoint, the first activematerial contained in the main negative electrode layer is selectedpreferably from lithium titanium composite oxides (excluding thosehaving a spinel structure), lithium molybdenum composite oxides, orlithium niobium composite oxides.

From the viewpoint of the reversibility of charging/discharging(charge/discharge cycle characteristics), the first active material isselected preferably from lithium titanium composite oxides having aramsdellite structure, an anatase structure, a rutile structure, abrookite structure or a bronze structure.

The lithium titanium composite oxide contained in the main negativeelectrode layer includes, for example, titanium oxides (for example,TiO₂) having an anatase structure, a rutile structure, a brookitestructure or a bronze structure, titanium oxides (for example,Li_(2+y)Ti₃O₇ [0≦y≦3]) having a ramsdellite structure, or those titaniumoxides obtained by substituting a part of their structural elements withother element(s). Examples of the titanium oxides include TiO₂ ortitanium-containing metal composite oxides containing Ti and at leastone element selected from the group consisting of P, V, Sn, Cu, Ni, Feand Co (for example, TiO₂—P₂O₅, TiO₂—V₂O₅, TiO₂—P₂O₅—SnO₂ andTiO₂—P₂O₅—MeO wherein Me is at least one element selected from the groupconsisting of Cu, Ni, Fe and Co). Such titanium-containing metalcomposite oxides preferably have a microstructure in which a crystalphase and an amorphous phase coexist or a single phase formed of anamorphous phase exists. The titanium-containing metal composite oxideshaving such a microstructure can attain a substantially high capacityeven upon high-rate charging/discharging and can outstandingly improvethe cycle performance.

The lithium absorption-release potential of any of such titanium oxidesis 1 to 2V (vs. Li/Li⁺).

Molybdenum oxides can be exemplified by Li_(x)MoO₂ (1 to 2V [vs.Li/Li⁺]) or Li_(x)MoO₃ (1 to 3V [vs. Li/Li⁺]). Niobium oxides can beexemplified by Li_(x)Nb₂O₅ (1 to 3V [vs. Li/Li⁺]). Their lithiumabsorption-release potentials are as shown in parentheses.

The first active material contained in the main negative electrode layeris preferably the titanium oxide having a ramsdellite structure or abronze structure, which is excellent particularly incharging/discharging reversibility (charge/discharge cyclecharacteristics). The first active material is most preferably thebronze-type titanium oxide having a largest electric capacity.

The first active material contained in the main negative electrode layeris desirably a particle having an average particle size of 3 μm or less,preferably 1 μm or less, and having a specific surface area in the rangeof 5 to 50 m²/g as determined by a BET method by N₂ adsorption. Thefirst active material particle having such average particle size andspecific surface area can improve their capacity factor and attain asubstantially high capacity even upon high-rate charging/discharging.The BET specific surface area by N₂ adsorption can be determined, forexample, with Micromeritex ASAP-2010 (manufactured by ShimadzuCorporation) using N₂ as absorption gas.

Generally, as the average particle size of the first active materialparticle is decreased that is, as the specific surface area of theactive material particles is increased, a battery made more excellent inlarge-current performance (output performance) is obtained, while acurrent passing therethough upon internal short-circuit is increasedthus significantly reducing the safety of the battery. The non-aqueouselectrolyte battery in accordance with the embodiment of the inventionforms a surface layer containing a specific second active material inthe main negative electrode layer as described above, thereby bringingabout an effect of suppressing internal short-circuit, and therefore,when the main negative electrode layer contains the first activematerial having a small average particle size and a large specificsurface area, the battery can simultaneously achieve large currentperformance (output performance) and high safety.

The main negative electrode layer contains a conductive material and abinder. As the conductive material, a carbon material for example can beused. Examples of the carbon material include acetylene black, carbonblack, cokes, carbon fibers and graphite. Other examples include metalpowder such as aluminum powder and electroconductive ceramics such asTiO. Particularly, cokes having an average particle size of 10 μm orless obtained by heat treatment at 800 to 2000° C., graphite, and carbonfibers having an average particle size of 1 μm or less are preferable.The BET specific surface area of the carbon material by N₂ adsorption ispreferably not lower than 10 m²/g. Examples of the binder includepolytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF),fluorine-based rubber, styrene-butadiene rubber, and a core-shellbinder.

Each proportion of the first active material, conductive material andbinder to be compounded in the main negative electrode layer ispreferably in the following range: the first active material being 70%by weight or more and 96% by weight or less, the conductive materialbeing 2% by weight or more and 28% by weight or less, and the binderbeing 2% by weight or more and 28% by weight or less. When the amount ofthe conductive material is less than 2% by weight, there is a fear thatthe current collective ability of the main negative electrode layer isdegraded and therefore, the large current performance of the non-aqueouselectrolyte battery is degraded. Also, when the amount of the binder isless than 2% by weight, the binding ability between the main negativeelectrode layer and the current collector is degraded and there istherefore a possibility that the cycle performance is degraded. On theother hand, each of the conductive material and the binder is preferably28% by weight or less from the viewpoint of obtaining a high capacity.

The porosity of the main negative electrode layer is preferably in therange of 20 to 50% by volume. The negative electrode including the mainnegative electrode layer of such porosity is made highly dense and isexcellent in affinity for the non-aqueous electrolyte. The porosity ismore preferably in the range of 25 to 40% by volume.

A copper foil, a nickel foil, a stainless steel foil, an aluminum foilor an aluminum alloy foil can be used as the current collector. When thelithium absorption-release potential of the active material contained inthe main negative electrode layer is 1V (vs. Li/Li⁺), an aluminum foilor an aluminum alloy foil is preferably used from the viewpoint oflightweight and the over-discharge resistance of the battery.

Preferably, the aluminum foil or aluminum alloy foil (current collector)has an average crystal grain size of 50 μm or less. Such a currentcollector can drastically increase strength so that the negativeelectrode can be made highly dense by high pressing pressure. As aresult, the battery capacity can be increased. Since the currentcollector can also be prevented from undergoing dissolution andcorrosion degradation in an over discharge cycle under a hightemperature (40° C. or more) environment, it is possible to suppress theelevation in the impedance of the negative electrode. Further, it ispossible to improve the output performance, the rapid chargingperformance, and the charge/discharge cycle performance of the battery.The average crystal grain size of the current collector is morepreferably 30 μm or less, even more preferably 5 μm or less.

The average crystal grain size can be determined in the followingmanner. The texture of the current collector surface is observed with anelectron microscope to determine the number (n) of crystal grainspresent within an area of 1 mm×1 mm. Then, the average crystal grainarea (S) is obtained from the equation: S=1×10⁶/n [μm²] wherein ndenotes the number of crystal grains noted above. Further, the averagecrystal grain size (d [μm]) is calculated from the obtained area S bythe following equation I:

d=2(S/π)^(1/2)   (I)

The aluminum foil or the aluminum alloy foil having an average crystalgrain size of 50 μm or less can be complicatedly affected by manyfactors such as the composition of the material, impurities, processconditions, history of the heat treatments and annealing conditions, andthe crystal grain size can be adjusted by an appropriate combination ofthe factors noted above during the manufacturing process.

The aluminum foil or the aluminum alloy foil has a thickness ofpreferably 20 μm or less, more preferably 15 μm or less. The aluminumfoil preferably has a purity of 99% by weight or more. The aluminumalloy is preferably an alloy containing another element such asmagnesium, zinc or silicon. On the other hand, the amount of thetransition metal such as iron, copper, nickel and chromium in thealuminum alloy is preferably 1% by weight or less.

The negative electrode is manufactured by suspending, for example, anactive material, a conductive material and a binder in a widely usedsolvent to prepare slurry, applying the slurry onto a current collector,drying it to form a main negative electrode layer, and then forming asurface layer on the main negative electrode layer by the methoddescribed above, followed by pressing the resultant layer.Alternatively, the main negative electrode layer may be prepared byforming an active material, a conductive material and a binder in theform of pellets and then forming them on the surface of a currentcollector.

(2) Outer Case

The outer case is formed of a laminate film having a thickness of 0.5 mmor less or a metallic vessel having a thickness of 1.0 mm or less. Thethickness of the metallic vessel is more preferably 0.5 mm or less.

The configuration of the outer case may be a flattened (thin-type), asquare type, a cylindrical type, a coin type or a button type. Theseouter cases may be variously designed depending on the size thereof. Forexample, it can be designed as an outer case for a small battery whichcan be mounted, for example, on mobile electronic instruments, or as anouter case for a large battery which can be mounted, for example, on amotor vehicle such as a two-wheeled vehicle or a four-wheeled vehicle.

The laminate film can make use of a multi-layer film having a metallayer interposed between resin layers. The metal layer is formedpreferably of aluminum foil or aluminum alloy foil for reducing theweight thereof. Examples of the resin layers include polymer materialssuch as polypropylene (PP), polyethylene (PE), nylon, or polyethyleneterephthalate (PET). The laminate film can be molded into any desiredconfiguration of outer case through sealing using thermal fuse-bonding.

The metal vessel is made of aluminum or aluminum alloys. The aluminumalloys preferably those containing an element such as magnesium, zinc,or silicon. If aluminum alloys containing a transition metal such asiron, copper, nickel or chromium are to be employed, the amount of thetransition metal is confined preferably to 100 ppm or less by weight.

(3) Positive Electrode

The positive electrode comprises a current collector and a positiveelectrode layer formed on one surface or both surfaces of the currentcollector and containing an active material, a conductive material and abinder.

Preferably the current collector is for example an aluminum foil or analuminum alloy foil containing an element such as Mg, Ti, Zn, Mn, Fe, Cuor Si.

The active material which can be used includes, for example, oxides,polymers.

The oxides which can be used include, for example, oxides having lithiumabsorbed therein, such as manganese dioxide (MnO₂), iron oxide, copperoxide or nickel oxide as well as lithium manganese composite oxides (forexample, Li_(x)Mn₂O₄ or Li_(x)MnO₂), lithium nickel composite oxides(for example, Li_(x)NiO₂), lithium cobalt composite oxides (for example,Li_(x)CoO₂), lithium nickel cobalt composite oxides (for example,LiNi_(1−y)Co_(y)O₂), lithium manganese cobalt composite oxides (forexample, Li_(x)Mn_(y)Co_(1−y)O₂), spinel type lithium manganese nickelcomposite oxides (for example, Li_(x)Mn_(2−y)Ni_(y)O₄), lithiumphosphorus oxide of olivine structure (for example, Li_(x)FePO₄,Li_(x)Fe_(1−y)Mn_(y)PO₄, Li_(x)CoPO₄), iron sulfate (Fe₂(SO₄)₃), orvanadium oxide (for example, V₂O₅). Herein, x and y are preferablyconfined to 0<x<1 and 0<y<1, respectively.

The polymers can be use, for example, conductive polymer materials suchas polyaniline or polypyrrole, as well as disulfide-based polymermaterials. It is also possible to use sulfur (S), carbon fluoride.

Preferable examples of the active materials include those exhibiting ahigher positive electrode voltage, such as lithium manganese compositeoxides (Li_(x)Mn₂O₄), lithium nickel composite oxides (Li_(x)NiO₂),lithium cobalt composite oxides (Li_(x)CoO₂), lithium nickel cobaltcomposite oxides (Li_(x)Ni_(1−y)Co_(y)O₂), spinel type lithium manganesenickel composite oxides (Li_(x)Mn_(2−y)Ni_(y)O₄), lithium manganesecobalt composite oxides (Li_(x)Mn_(y)Co_(1−y)O₂), or lithium ironphosphate (Li_(x)FePO₄). Herein, x and y are preferably confined to0<x≦1 and 0≦y≦1, respectively.

Most preferable examples of the active materials include lithium cobaltcomposite oxides or lithium manganese composite oxides. These compositeoxides are characterized by high ionic conductivity, so that when theyare used in combination with the negative-electrode active materials inthe embodiment of the invention, the diffusion of lithium ions in theactive materials can hardly serve as a rate determining step. For thisreason, these composite oxides are excellent in compatibility with thelithium titanium composite oxides (excluding those having a spinelstructure) that are the active material in the main negative electrodein the embodiment.

Preferably, the primary particle size of the active material is confinedto 100 nm or more and 1 μm or less. By doing so, the handling of theactive material would become easier in the industrial production, andthe inter-solid diffusion of lithium ions can proceed smoothly.

Preferably, the specific surface area of the active material is confinedto 0.1 to 10 m²/g. By doing so, it becomes possible to sufficientlysecure the absorption/release site of lithium ions and the handlingthereof would become easier in the industrial production, and it ispossible to secure excellent charge/discharge cycle performance.

The conductive material can use, for example, carbonaceous materialssuch as acetylene black, carbon black, or graphite. These conductivematerials are useful in enhancing the electronic collecting performanceand for suppressing the contact resistance of the active material to thecurrent collector.

The binder for binding the active material to the conductive materialcan use, for example, polytetrafluoroethylene (PTFE), polyvinylidenefluoride (PVdF), or fluorine-based rubber.

The mixing ratio among the active material, the conductive material andthe binder is preferably 80 to 95% by weight of the active material, 3to 18% by weight of the conductive material, and 2 to 17% by weight ofthe binder. When the mixing ratio of the conductive material is not lessthan 3% by weight, it is possible to exhibit the aforementioned effects.When the mixing ratio of the conductive material is confined to 10% byweight or less, it is possible to minimize the decomposition ofnon-aqueous electrolyte on the surface of the conductive material evenin high-temperature storage. When the binder is incorporated at a mixingratio of not less than 2% by weight, it is possible to secure asufficient strength of the electrode. When the mixing ratio of thebinder is confined to 10% by weight or less, it is possible to decreasethe mixing ratio of the insulating material of the electrode and todecrease the internal resistance of the electrode.

The positive electrode is manufactured for example by suspending theactive material, the conductive material and the binder in a suitablesolvent to prepare slurry, then applying the slurry onto the surface ofa current collector and drying it to form a positive electrode layer,followed by pressing the resultant layer. Alternatively, a mixtureconsisting of an active material, a conductive material and a binder maybe formed into pellets and used to form the positive electrode layer.

(4) Non-Aqueous Electrolyte

Examples of the non-aqueous electrolyte include a liquid-formnon-aqueous electrolyte prepared by dissolving an electrolyte in anorganic solvent, or a gel-form non-aqueous electrolyte prepared bycompounding the liquid-form non-aqueous electrolyte and a polymermaterial.

The liquid-form non-aqueous electrolyte is prepared by dissolving anelectrolyte in a concentration of 0.5 to 2.5 mol/L in an organicsolvent.

Examples of the electrolyte include lithium salts such as lithiumperchlorate (LiClO₄), lithium hexafluorophosphate (LiPF₆), lithiumtetrafluoroborate (LiBF₄), lithium hexafluoroarsenate (LiAsF₆), lithiumtrifluoromethanesulfonate (LiCF₃SO₃) and bistrifluoromethylsulfonylimidelithium (LiN(CF₃SO₂)₂), or mixtures of these compounds. Those which arescarcely oxidized at a high potential are preferable and LiPF₆ is mostpreferable.

Examples of the organic solvent that can be used include single or mixedsolvents of cyclic carbonates such as propylene carbonate (PC), ethylenecarbonate (EC) or vinylene carbonate; linear carbonates such as diethylcarbonate (DEC), dimethyl carbonate (DMC) or methyl ethyl carbonate(MEC); cyclic ethers such as tetrahydrofuran (THF),2-methyltetrahydrofuran (2MeTHF) or dioxolan (DOX); linear ethers suchas dimethoxyethane (DME) or diethoxyethane (DEE); γ-butyrolactone (GBL);acetonitrile (AN); or sulfolane (SL).

Examples of the polymer material may include polyvinylidene fluoride(PVdF), polyacrylonitrile (PAN) or polyethylene oxide (PEO).

The organic solvent is preferably a mixed solvent containing two or morekinds of solvents selected from the group consisting of propylenecarbonate (PC), ethylene carbonate (EC) and γ-butyrolactone (GBL). Morepreferable organic solvent is γ-butyrolactone (GBL). The reason for thisis as follows.

The lithium composite oxide of titanium, molybdenum or niobium performsabsorption-release of lithium ions in the potential range of about 1 to3V (vs. Li/Li⁺). However, reductional decomposition of a non-aqueouselectrolyte hardly occurs in this potential range, and a coating that isa reduced product of a non-aqueous electrolyte is less likely to beformed on the surface of the lithium titanium composite oxide. For thesereasons, when the lithium titanium composite oxide is preserved in alithium-absorbed state i.e. in a charged state, the lithium ionsabsorbed into the lithium titanium composite oxide gradually diffuseinto the electrolyte, whereupon so-called self-discharge is induced.Self-discharge noticeably takes place when an environment where thebattery is stored becomes hot.

Among the aforementioned organic solvents, the γ-butyrolactone is moreliable to reduction as compared with linear carbonates or cycliccarbonates. Specifically, the order of liability to reduction is:γ-butyrolactone>>>ethylene carbonate>propylene carbonate>>dimethylcarbonate>methylethyl carbonate>diethyl carbonate. Therefore, when thisγ-butyrolactone is incorporated into an electrolyte, it becomes possibleto form an excellent film on the surface of the lithium titaniumcomposite oxide even at the working potential range of the lithiumtitanium composite oxide. As a result, it is possible to suppress theself-discharging and to enhance the high-temperature storagecharacteristics of the non-aqueous electrolyte battery.

Even in the case of the aforementioned mixed solvent including two ormore solvents selected from the group consisting of propylene carbonate(PC), ethylene carbonate (EC) and γ-butyrolactone (GBL) or especially inthe case of a mixed solvent containing γ-butyrolactone, it is possibleto suppress the self-discharging likewise and to enhance thehigh-temperature storage characteristics of the non-aqueous electrolytebattery.

γ-Butyrolactone is preferable because it can form an excellentprotective film by incorporating it in an amount of 40 to 95% by volumebased on the organic solvent.

(5) Separator

Examples of the separator include a porous film formed of polyethylene,polypropylene, cellulose or polyvinylidene fluoride (PVdF), and unwovenfabrics formed of synthetic resin. Among them, a porous film formed ofpolyethylene or polypropylene is preferable, since it is capable ofbeing fused at a predetermined temperature, thereby cutting of electriccurrent. Thus, this porous film is preferable from the viewpoint ofenhancing the safety.

Next, the non-aqueous electrolyte battery (a flattened non-aqueouselectrolyte battery where the outer case is formed of a laminate film)according to the embodiment of the invention will be explained withreference to FIGS. 1 and 2. FIG. 1 is a cross-sectional viewillustrating a flattened non-aqueous electrolyte battery and FIG. 2 isan enlarged cross-sectional view of part A in FIG. 1. These figures areschematically illustrated for the purpose of explanation and forfacilitating the understanding of the present invention, so that theconfiguration, dimension and dimensional ratio thereof may differ fromthose of the actual device. These features and design can be optionallymodified by referring to the following description and the technologiesknown in the art.

A flattened wound electrode group 1 is housed in a bag-like outer case 2which is formed from a laminate film including a couple of resin layerswith aluminum foil interposed between them. The flattened woundelectrode group 1 is constructed by spirally wounding and press-moldinga laminate including mentioning from outside, a negative electrode 3, aseparator 4, a positive electrode 5 and a separator 4. As shown in FIG.2, the negative electrode 3 constituting the outermost husk is composedof a current collector 3 a, a main negative electrode layer 3 b formedon one side of the current collector 3 a, and a surface layer 3 c formedon the main negative electrode layer 3 b. The other negative electrode 3is composed of a current collector 3 a, a main negative electrode layer3 b and a surface layer 3 c formed respectively on both sides of thecurrent collector 3 a. The surface layer 3 c contains the specificsecond active material described above, for example, the spinel typelithium titanium composite oxide. The main negative electrode layer 3 bcontains a first active material different from the spinel type lithiumtitanium composite oxide, for example, selected from carbonaceousmaterials, lithium titanium composite oxides (excluding those having aspinel structure), lithium molybdenum composite oxides, and lithiumniobium composite oxides. The positive electrode 5 is constructed suchthat a positive electrode layer 5 b is formed on both sides of apositive electrode current collector 5 a.

In the vicinity of the outer circumferential edge portion of the woundelectrode group 1, a negative electrode terminal 6 is electricallyconnected to the current collector 3 a of the negative electrode 3constituting the outermost husk, and a positive electrode terminal 7 iselectrically connected to the positive electrode collector 5 a of theinner positive electrode 5. The negative electrode terminal 6 andpositive electrode terminal 7 are led out through an opening of thebag-like outer case 2. For example, a liquid non-aqueous electrolyte ispoured into the bag-like outer case 2 through the opening of the outercase 2. The opening of the bag-like outer case 2 is heat-sealed with thenegative electrode terminal 6 and positive electrode terminal 7 beinglocated inside, thereby completely sealing the wound electrode group 1and the liquid non-aqueous electrolyte.

The negative terminal can make use of electrically conductive materialshaving electric stability and conductive property under the conditionwhere the electric potential thereof to lithium ion metal is in therange of 1.0 to 3.0V. Examples of the material for the negative terminalinclude aluminum and aluminum alloys containing elements such as Mg, Ti,Zn, Mn, Fe, Cu, and Si. Preferably, the negative terminal is formed ofthe same material as that of the current collector of the negativeelectrode in order to minimize the contact resistance thereof to thecurrent collector of the negative electrode.

The positive terminal can make use of electrically conductive materialshaving electric stability and conductive property under the conditionwhere the electric potential thereof to lithium ion metal is in therange of 3.0 to 4.25V. Examples of the material for the positiveterminal include aluminum and aluminum alloys containing elements suchas Mg, Ti, Zn, Mn, Fe, Cu, and Si. Preferably, the positive terminal isformed of the same material as that of the current collector of thepositive electrode in order to minimize the contact resistance thereofto the current collector of the positive electrode.

According to the non-aqueous electrolyte battery in accordance with suchembodiments, the surface layer in the negative electrode contains asecond active material (for example, a lithium titanium composite oxidehaving a spinel structure) that absorbs and releases lithium and has avolume resistivity of 1×10⁵ Ωcm or more in a lithium-nonabsorbed state,and the volume resistivity of the second active material in alithium-absorbed state is 1×10⁻² times or less relative to the volumeresistivity thereof in a lithium-nonabsorbed state, thereby improvingsafety at the time of internal short-circuit without reducing largecurrent performance, owing to the action described above.

Now, the battery pack according to the embodiment of the invention isdescribed in detail.

The battery pack according to the embodiment is provided with aplurality of the aforementioned non-aqueous electrolyte batteries(single cells) which are electrically connected to one another inseries, in parallel, or in series and parallel.

In the battery pack according to the embodiment, a plurality of theaforementioned non-aqueous electrolyte batteries with improvement insafety upon internal short-circuit are formed as single cells into anassembled battery, so that high safety can be maintained.

The battery pack in accordance with the embodiment will be explained indetail with reference to FIGS. 3 and 4. The single cell can make use ofthe flattened battery shown in FIG. 1.

A plurality of single cells 21 each being formed of the flattenednon-aqueous electrolyte battery shown in FIG. 1 are laminated such thatthe negative electrode terminal 6 and the positive electrode terminal 7,both being externally led out, are arrayed to extend in the samedirection, and the single cells are clamped together by means of anadhesive tape 22, thereby creating a assembled battery 23. These singlecells 21 are electrically connected to one another in series as shown inFIG. 4.

A printed wiring board 24 is disposed to face the side wall of each ofthe single cells 21 where the negative electrode terminal 6 and thepositive electrode terminal 7 are externally led out. On this printedwiring board 24 are mounted a thermistor 25, a protection circuit 26,and a terminal 27 for electrically connecting the printed wiring board24 to external instruments. It should be noted that in order to preventunwanted electric connection to the wirings of the assembled battery 23,an insulating plate (not shown) is attached to the surface of theprotection circuit board 26 that faces the assembled battery 23.

A lead 28 for the positive electrode is electrically connected, throughone end thereof, to the positive electrode terminal 7 which is locatedat the lowest layer of the assembled battery 23. The other end of thelead 28 is inserted into, and electrically connected to, a connector 29for the positive terminal of the printed wiring board 24. A lead 30 forthe negative electrode is electrically connected, through one endthereof, to the negative electrode terminal 6 which is located at thehighest layer of the assembled battery 23. The other end of the lead 30is inserted into, and electrically connected to, a connector 31 for thenegative terminal of the printed wiring board 24. These connectors 29and 31 are electrically connected, through interconnects 32 and 33formed on the printed wiring board 24, to the protection circuit 26.

The thermistor 25 is used for detecting the temperature of single cells21 and the signals thus detected are transmitted to the protectioncircuit 26. The protection circuit 26 is designed to cut off, underprescribed conditions, a wiring 34 a of plus-side and a wiring 34 b ofminus-side which are interposed between the protection circuit 26 andthe terminal 27 for electrically connecting the printed wiring board 24to external instruments. The expression “under prescribed conditions”herein means the conditions under which the temperature detected by thethermistor 25 becomes higher than a predetermined temperature forexample. Further, the expression “under prescribed conditions” hereinalso means the conditions under which the over-charging,over-discharging and over-current of the single cells 21 are detected.This over-charging is detected for the single cells 21 individually orentirely. In the case where the single cells 21 are to be detectedindividually, either the voltage of the cell may be detected or thepotential of the positive or negative electrode may be detected. In thelatter case, a lithium electrode is inserted, as a reference electrode,into individual cells 21. In the case of the battery pack shown in FIGS.3 and 4, a wiring 35 is connected to each of the single cells 21 fordetecting the voltage thereof, and the signals detected are transmitted,through this wiring 35, to the protection circuit 26.

On all of the sidewalls of the assembled battery 23 excluding onesidewall where the positive electrode terminal 7 and the negativeelectrode terminal 6 are protruded, a protective sheet 36 made of rubberor synthetic resin is disposed respectively.

The assembled battery 23 is housed, together with protective sheets 36and the printed wiring board 24, in a case 37. Namely, the protectivesheets 36 are disposed respectively on the opposite inner sidewallsconstituting the longer sides of the case 37 and on one inner sidewallconstituting one shorter side of the case 37. The printed wiring board24 is disposed on the other sidewall constituting the other shorter sideof the case 37. The assembled battery 23 is positioned in a spacesurrounded by the protective sheets 36 and the printed wiring board 24.A lid 38 is attached to the top of the case 37.

A thermally shrinkable tube may be used in place of the adhesive tape 22in order to fix the assembled battery 23. In this case, the protectivesheet is disposed on the opposite sidewalls of the assembled battery andthen the thermally shrinkable tube is disposed to surround theseprotective sheets, after which the thermally shrinkable tube is allowedto thermally shrink, thereby fastening the assembled battery.

In the embodiment shown in FIGS. 3 and 4, the single cells 21 areelectrically connected to one another in series. However, the singlecells may be electrically connected to one another in parallel or bothin series and in parallel in order to increase the capacity of thebattery. A plurality of battery packs, each being assembled as describedabove, may further be electrically connected to one another in series orin parallel.

Further, the embodiment of the battery pack may be optionally modifieddepending on the end-use thereof. As for the end-use of the batterypack, it can be preferably applied to those where excellent cyclecharacteristics are desired in large current performance. Specifically,the battery pack can be employed as a power source for digital camerasor as an on-vehicle type power source for two-wheeled or four-wheeledhybrid electric vehicles, for two-wheeled or four-wheeled electricvehicles, or for electrically assisted bicycles. The battery pack isparticularly suited for use as an on-vehicle type power source.

As described above, the non-aqueous electrolyte containing a mixedsolvent including two or more kinds of solvents selected from the groupconsisting of propylene carbonate (PC), ethylene carbonate (EC) andy-butyrolactone (GBL), or the non-aqueous electrolyte containingγ-butyrolactone (GBL), can be used to obtain a non-aqueous electrolytebattery excellent in high-temperature characteristics. A battery packprovided with a assembled battery consisting of a plurality ofnon-aqueous electrolyte batteries each constructed as described above isespecially suited for use as an on-vehicle type power source.

Hereinafter, examples of the present invention will be explained.However, the following examples are not intended to limit the scope ofthe invention unless the gist of the invention is exceeded.

EXAMPLE 1 <Preparation of Positive Electrode>

First, 90% by weight of lithium manganese oxide (LiMn_(1.9)Al_(0.1)O₄)powder having a spinel structure as an active material, 5% by weight ofacetylene black as a conductive material, and 5% by weight ofpolyvinylidene fluoride (PVdF) were added to and mixed withN-methylpyrrolidone (NMP) to prepare slurry. This slurry was thenapplied onto both surfaces of a current collector made of an aluminumfoil having a thickness of 15 μm, then dried and pressed to prepare apositive electrode having an electrode density of 2.9 g/cm³.

<First Active Material in Negative Electrode >

As the first active material, a mesophase pitch based carbon fiberheat-treated at 3,000° C. (fiber diameter, 8 μm; average fiber length,20 μm; average spacing [d₀₀₂], 0.3360 nm) was used.

<Synthesis of Second Active Material in Negative Electrode>

Li₂CO₃ and anatase-type TiO₂ were mixed at a Li/Ti molar ratio of 4/5and then baked at 850° C. for 12 hours in the atmosphere and then groundto yield a spinel-type lithium titanium composite oxide (Li₄Ti₅O₁₂)particle having an average particle size of 0.8 μm as a second activematerial.

The average particle size of the spinel-type lithium titanium compositeoxide particle was measured by the following method using a laserdiffraction type distribution measuring instrument (SALD-300manufactured by Shimadzu Corporation). First, about 0.1 g of activematerial sample, surface active agent, and 1 to 2 mL of distilled waterwere added in a beaker, and stirred and mixed sufficiently. This mixedsolution was then poured into an agitating water tank, and the luminousintensity distribution was measured 64 times at intervals of 2 secondsto numerically analyze the particle size distribution data, therebydetermining the average particle size of the spinel-type lithiumtitanium composite oxide.

<Preparation of Negative Electrode>

95% by weight of the first active material and 5% by weight ofpolyvinylidene fluoride (PVdF) were added to and mixed withN-methylpyrrolidone (NMP) to prepare slurry. This slurry was thenapplied onto both sides of a current collector made of a copper foilhaving a thickness of 12 μm, and then dried to prepare a main negativeelectrode layer. Then, 90% by weight of the second active material, 5%by weight of acetylene black as a conductive material, and 5% by weightof polyvinylidene fluoride (PVdF) were added to and mixed withN-methylpyrrolidone (NMP) to prepare slurry. This slurry was thenapplied onto the surface of the main negative electrode layer and thendried to form a surface layer. Thereafter, it was pressed to manufacturea negative electrode having an electrode density of 1.7 g/cm³.

The surface layer and main negative electrode layer of the resultingnegative electrode were cut respectively, and with SEM-EDX, theresulting sections were measured for their Ti concentration in adirection perpendicular to the current collector. As a result, theaverage thickness of the surface layer was about 10 μm. It was alsoconfirmed that the surface layer is present in uniform thickness oversubstantially the whole area of the main negative electrode layer. Theaverage thickness of the main negative electrode layer was 30 μm.

<Preparation of Electrode Group>

The positive electrode, a separator made of a porous polyethylene filmhaving a thickness of 25 μm, the negative electrode, and anotherseparator were laminated in the mentioned order to form a flattenedelectrode group. The resulting electrode group was housed in a pack madeof an aluminum laminate film and then vacuum dried at 80° C. for 24hours.

<Preparation of Liquid Non-Aqueous Electrolyte>

LiBF₄ was dissolved as an electrolyte to a concentration of 1.5 mol/L ina mixed solvent consisting of ethylene carbonate (EC) andγ-butyrolactone (GBL) at a volume ratio of 1/2, to prepare a liquidnon-aqueous electrolyte.

The liquid non-aqueous electrolyte was injected into the laminate filmpack in which the electrode group had been housed, and the pack wascompletely sealed by heat-sealing, to produce a non-aqueous electrolytesecondary battery with the structure shown in FIG. 1, having a width of70 mm, a thickness of 6.5 mm, a height of 120 mm and a capacity of 3 Ah.

COMPARATIVE EXAMPLE 1

A non-aqueous electrolyte secondary battery was produced in the samemanner as in Example 1 except that a negative electrode having a mainnegative electrode layer only formed on a current collector was used.

COMPARATIVE EXAMPLE 2

A non-aqueous electrolyte secondary battery was produced in the samemanner as in Example 1 except that an alumina layer of 10 μm inthickness consisting of alumina (Al₂O₃) having an average particle sizeof 0.1 μm was formed in place of the surface layer containing the secondactive material.

COMPARATIVE EXAMPLE 3

A non-aqueous electrolyte secondary battery was produced in the samemanner as in Example 1 except that a surface layer containing titania(rutile-type TiO₂) having an average particle size of 0.1 μm in place ofthe second active material was formed on a main negative electrodelayer.

The batteries in Example 1 and Comparative Examples 1 to 3 wereevaluated for their load characteristics at 1 C and 30 C. Thereafter,each battery was charged to 4.4V and then crushed with round bar havingdiameter of 1 cm, and the surface temperature of the battery wasmeasured. The test was carried out twice using different batteries. Theresults are shown in Table 1.

TABLE 1 Load characteristic Maximum Maximum (30 C/1 C temperature oftemperature of capacity ratio) battery (first time) battery (secondtime) Example 1 80%  91° C.  97° C. Comparative 80% 253° C. 298° C.Example 1 Comparative 50% 168° C. 187° C. Example 2 Comparative 50% 171°C. 188° C. Example 3

As shown in Table 1 above, the battery in Example 1 showed minimum heatgeneration and was thus confirmed to have high safety. The loadcharacteristics in Example 1, wherein a spinel-type lithium titaniumoxide was contained in a surface layer, were equivalent to those inComparative Example 1 wherein a surface layer-free negative electrodehaving a main negative electrode layer only formed on a currentcollector was used.

On the other hand, the battery in Comparative Example 2 wherein analumina layer was formed on a main negative electrode layer, and thebattery in Comparative Example 3 wherein a titania-containing surfacelayer was formed on a main negative electrode layer, showedsignificantly reduced load characteristics as compared with those inExample 1 and Comparative Example 1.

EXAMPLE 2

The positive and negative electrodes prepared in Example 1 were opposedto each other via a separator provided with an 18 mm square internalshort-circuit area to prepare a single layer battery.

COMPARATIVE EXAMPLE 4

The positive and negative electrodes prepared in Comparative Example 1were opposed to each other via a separator provided with an 18 mm squareinternal short-circuit area to prepare a single layer battery.

EXAMPLES 3 TO 8

Single layer batteries were prepared in the same manner as in Example 2except that the first active material contained in the main negativeelectrode layer was changed to the materials shown in Table 2 below.

COMPARATIVE EXAMPLES 5 TO 8

Single layer batteries were prepared in the same manner as inComparative Example 4 except that the first active material contained inthe main negative electrode layer was changed to the materials shown inTable 2 below.

The internal short-circuit area of each of the single layer batteriesobtained in Examples 2 to 8 and Comparative Examples 4 to 8 was pressedat a pressure of about 7 N/cm², during which a change in voltage wasmonitored. The time elapsed until the voltage of each battery arrived at1V is shown in Table 2.

TABLE 2 Main negative electrode layer Thickness Particle Specific ofsurface Arrival First active size surface layer time material (μm) area(m²/g) (μm) (min) Comparative Carbon fiber 8 1 None 0.2 Example 4Example 2 Carbon fiber 8 1 10 100 Example 3 Carbon fiber 8 1  1 90Example 4 Carbon fiber 8 1  3 100 Comparative TiO₂ 0.6 20 None <3Example 5 (bronze type) Example 5 TiO₂ 0.6 20 10 >100 (bronze type)Comparative Li₂Ti₃O₇ 0.8 10 None <3 Example 6 Example 6 Li₂Ti₃O₇ 0.8 1010 >100 Comparative Li_(x)MoO₂ 3 5 None <3 Example 7 Example 7Li_(x)MoO₂ 3 5 10 >100 Comparative Li_(x)Nb₂O₅ 3 5 None <3 Example 8Example 8 Li_(x)Nb₂O₅ 3 5 10 >100

As is evident from Table 2, it can be seen that upon internalshort-circuit, the voltage reduction of the batteries in Examples 2 to 8is slower than in the batteries in Comparative Examples 4 to 8. This isconsidered due to the fact that the spinel-type lithium titaniumcomposite oxide that is the second active material contained in thesurface layer suppresses the flow of internal short-circuit current.

Accordingly, the batteries in Examples 2 to 8 are low in heat generationupon internal short-circuit and can secure high safety.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A non-aqueous electrolyte battery comprising: an outer case; apositive electrode housed in the outer case; a negative electrode housedin the outer case with a space from the positive electrode; and anon-aqueous electrolyte accommodated in the outer case, wherein thenegative electrode comprises a current collector and a negativeelectrode layer formed on one surface or both surfaces of the currentcollector, and the negative electrode layer includes at least one mainnegative electrode layer which is formed on the surface of the currentcollector and contains a first active material, and a surface layerwhich is formed on the surface of the main negative electrode layer andcontains a second active material different from the first activematerial, the second active material being a lithium titanium compositeoxide having a spinel structure.
 2. The battery according to claim 1,wherein the lithium titanium composite oxide having a spinel structureis Li_(4+x)Ti₅O₁₂ (−1≦x≦3).
 3. The battery according to claim 1, whereinthe surface layer has an average thickness of 3 to 30 μm.
 4. The batteryaccording to claim 1, wherein the surface layer further comprises aconductive material.
 5. The battery according to claim 1, wherein anabsorption/release potential of the first active material is 1 to 3V(vs. Li/Li⁺).
 6. The battery according to claim 1, wherein the firstactive material is selected from a group consisting of carbonaceousmaterials, lithium titanium composite oxides excluding those having aspinel structure, lithium molybdenum composite oxides, and lithiumniobium composite oxides.
 7. The battery according to claim 1, whereinthe first active material is selected from lithium titanium compositeoxides having a ramsdellite structure, an anatase structure, a rutilestructure, a brookite structure or a bronze structure.
 8. The batteryaccording to claim 1, wherein the specific surface area of the firstactive material is not less than 5 m²/g.
 9. The battery according toclaim 1, wherein the first active material is a particle having theaverage particle size of 0.1 to 3 μm.
 10. The battery according to claim1, wherein the capacity of the battery is 3 Ah or more.
 11. Anon-aqueous electrolyte battery comprising: an outer case; a positiveelectrode housed in the outer case; a negative electrode housed in theouter case with a space from the positive electrode; and a non-aqueouselectrolyte accommodated in the outer case, wherein the negativeelectrode comprises a current collector and a negative electrode layerformed on one surface or both surfaces of the current collector, thenegative electrode layer includes at least one main negative electrodelayer which is formed on the surface of the current collector andcontains a first active material, and a surface layer which is formed onthe surface of the main negative electrode layer and contains a secondactive material different from the first active material, the secondactive material absorbs and releases lithium, a volume resistivity ofthe second active material in a lithium-nonabsorbed state is 1×10⁵ Ωcmor more, and the volume resistivity of the second active material in alithium-absorbed state is 1×10⁻² times or less relative to the volumeresistivity thereof in a lithium-nonabsorbed state.
 12. The batteryaccording to claim 11, wherein the volume resistivity of the secondactive material in the lithium-absorbed state is 1×10⁻⁴ times or lessrelative to the volume resistivity thereof in the lithium-nonabsorbedstate.
 13. A battery pack comprising a plurality of the non-aqueouselectrolyte batteries of claim 1, which are electrically connected eachother in series, in parallel, or in series and parallel.
 14. The batterypack according to claim 13, which further comprises a protection circuitcapable of detecting the voltage of the batteries.